The present invention pertains to a manually operated, engine driven vibratory concrete screed and, more particularly, to an improved vibration isolation and control system for such a screed.
Vibratory screeds are used to smooth the surface of freshly poured concrete and eliminate air pockets within the concrete mass. One type of manually operated screed is driven by a small gasoline engine (e.g. 1 to 2.5 horsepower) that turns an eccentric exciter mechanism to impart a high speed vibratory force to a screed blade attached to the exciter mechanism. For example, an engine operating in the range of 5,000–7,500 rpm will generate in a centrifugal force in the range of about 245 lbs. to 550 lbs. This type of vibratory screed includes an operating handle connected through a frame piece to the vibratory exciter and engine. The machine is pulled over the surface of the concrete and a small amount of fresh concrete will build-up behind the blade to ensure that the surface is uniform and depressions are not created. The blade may be up to 24 feet in length and, although vibration of the blade helps make the concrete flow, the operator must still pull the machine. When the build-up of concrete behind the blade is uneven, there is a tendency for one end of the blade to lift and create an uneven surface. The operator must tilt the operating handle downwardly on one side to generate a force sufficient to counteract the upward movement of the blade. This requires the operator to exert a large amount of force on the handle. Also, the screed blade may have to be turned horizontally over the surface of the concrete, as when moving around a curve or a corner, requiring the operator to exert a large amount of force on the handle in a generally horizontal plane.
It is also necessary to isolate the transmission of vibration from the exciter and blade to the operator. Specifically, the frame that carries the operator handle is isolated from its connection to the blade or to the exciter mechanism with rubber or other elastomer vibration isolators. It is desirable to use as soft a vibration isolator as possible to provide maximum vibration isolation for the operator. However, because of the high loads that the operator must impose on the blade for the reasons discussed above, harder vibration isolators are required in order to provide an adequately stiff connection between the operator handle and the blade to transmit the required control force. Soft vibration isolators, e.g. those having a durometer of about 30 provide excellent vibration isolation for the operator, but are too soft to permit adequate force to be transmitted from the handle, through the isolators, to the blade. Soft isolators also amplify the distance through which the operator must move the operating handle to adequately control the blade. The operator handle may be as much as 3.5 feet from the vibration isolators such that a very small amount of movement at the isolator connection is magnified into a large amount of movement where the operator grasps the operating handle.